your using mental ray and you dont know where your images went?? try something like defaulting your preferances, use .tiff, use maya software, and look in the 'images' folder. try something a little less than 4K..,maybe 610 by 512 (whatever it is)??!
4K and mental ray?? no doubt you are using ray tracing too?? I think you'd need a quad processor.., or you will be waiting until next year for the render.
take it easy and life will be easy